Take a look at this video of a waiting room. Do you see anything strange?
Perhaps you saw the rug disappear, or the couch pillows transform, or a few ceiling panels evaporate. Or maybe you didn’t. In fact, dozens of objects change in this video, which won second place in the Best Illusion of the Year Contest in 2021. Voting for the latest version of the contest opened on Monday.
Illusions “are the phenomena in which the physical reality is divorced from perception,” said Stephen Macknik, a neuroscientist at SUNY Downstate Health Sciences University in Brooklyn. He runs the contest with his colleague and spouse, Susana Martinez-Conde.
By studying the disconnect between perception and reality, scientists can better understand which brain regions and processes help us interpret the world around us. The illusion above highlights change blindness, the brain’s failure to notice shifts in the environment, especially when they occur gradually.
To some extent, all sensory experience is illusory, Dr. Martinez-Conde asserts.
“We are always constructing a simulation of reality,” she said. “We don’t have direct access to that reality. We live inside the simulation that we create.”
She and Dr. Macknik have run the illusion contest since 2005. What began as a public outreach event at an academic conference has since blossomed into an annual competition open to anyone in the world.
They initially worried that people would run out of illusions to submit. “But that actually never happened,” Dr. Martinez-Conde said. “What ended up happening instead is that people started developing illusions, actually, with an eye to competing in the contest.”
We encounter illusions in many ways. Some appear in everyday life, when the observer notices that what is seen doesn’t correspond with what is known to be real. Aristotle noted one of the oldest known illusions: After gazing at a flowing stream for some time, he noticed that stationary rocks on the opposite bank appeared to be moving upstream.
This phenomenon, known as the “waterfall illusion,” or motion aftereffect, occurs when neurons in the viewer’s brain adapt to the sense of motion in one direction, which biases perception of motion in the opposite direction, Dr. Macknik explained.
Other illusions are created intentionally. The blue rows in this image, which appear to slant, are a variant of an older illusion discovered on the wall of a cafe in the 1970s. The illusion above, which won second prize in the contest in 2017, illustrates how certain shapes can distort the appearance of others while electrical signals in the brain interfere with each other.
This image of a star, a finalist in the 2012 contest, is a revamp of prior illusions showing peripheral drift, in which something static appears to move. Peripheral drift happens when your eyes blink or involuntarily shift while viewing an image with a distinct color pattern, but stops when you fix your gaze on a single point.
With their knowledge of the brain, neuroscientists can even anticipate illusions. In 1998, Dr. Macknik accurately predicted that the center bar in the illusion below would disappear if the outer bars came too close to one another. That’s because, when the bars are close together, the neurons in your brain that perceive the outer bars can mask signals from the neurons that perceive the center bar This effect becomes more pronounced when looking at the bars with your peripheral vision.
“This means that it is the distance in your mind that matters, not real space,” Dr. Macknik said.
Any of our five senses can generate an illusion. A pungent odor that fades after a while, simply because your nose has adapted to it, is an example of an olfactory illusion.
Most of the illusions submitted to the contest are related to visual perception. That’s partly because it is held online, a realm beyond taste, touch and smell. But the contest also reflects how much of the human brain is devoted to making sense of what we see.
“The more processing power you have, I believe, the more illusions you incur,” Dr. Martinez-Conde said.
She and Dr. Macknik described perception as a bag of tricks that our brains, which have evolved to work quickly, utilize to navigate the world efficiently. But that approach to perception has limits, and illusions are the evidence. They are “cracks of the plaster” of the universe that our brains construct for us, Dr. Macknik said.
A toy car that appears to drive through a paper wall illustrates how our brains use contextual cues to create three-dimensional images. Fake mirror reflections, like the one below, reveal that our brains, too, make use of Occam’s razor.
Illusions persist even in space, as in this image of lunar craters that to some viewers may instead look like bumps. Dr. Martinez-Conde saw craters initially but later saw bumps. Dr. Macknik only ever saw the craters. “What illusion?” he said.
The brain has tricks indeed.
Katrina Miller is a science reporter for The Times based in Chicago. She earned a Ph.D. in physics from the University of Chicago.
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